The present invention relates to a digital wireless communications system comprising a first wireless radio station coupled to a network and a second wireless radio station arranged for wireless communication with the first wireless station via at least one time slot of a communications frame, in which system the first station comprises a master clock and is arranged to transmit messages to the second station comprising a synchronisation pattern, and in which system the second station comprises power management circuitry, local timing circuitry, and is at least controllable so as to operate in an active reception mode and in a power down mode, whereby the power down mode is adopted between reception of messages outside the active mode. Such a digital wireless communications system can be a FD/TDMA cordless telephony system comprising cordless handsets as wireless radio stations, or any other suitable digital wireless communication system in which date are exchanged via time slots in transmission frames.
The present invention further relates to a wireless radio station for use in such a system.
A digital wireless communication system of the above kind is known from the Philips Data Handbook IC17, "Semiconductors/Wireless Communications", 1996, pp. 34-37. On page 37, a blockdiagram of a DECT handset is shown. This handset comprises a double superheterodyne receiver for receiving message from the cordless base station shown on page 36. Furthermore, so-called Burst Mode Logic is shown for exchanging bursts of information between a common data memory and the transceiver part. The burst are transmitted and received in time slots of communications frames. The handset synchronizes to synchronisation patterns send by the base station, the base station acting as a master station and the handset acting a slave station. The handset can adopt an active mode and a power down or sleep mode, inter alia. The shown power management block controls various parts of the handset to be switched off when the handset adopts the power down mode, such a power management being well known in the art. The so-called ABC (ADPCM-Codec, Burst Mode Controller, and microController) chip of Philips type PCD509x shown on page 37 is readily available onto the market. In DECT, the base station transmits paging messages for informing handsets that there is an incoming call. In order that the handset becomes aware of such paging messages when in power down mode and also to keep in synchronisation with the base station it is necessary that the handset's receiver is switched on for receiving these paging messages and for synchronisation purposes. Local timing circuitry controls switching on of the receiver, inter alia. The synchronisation pattern in the paging message is used to undo an accumulated timing error between the reception of paging messages. The timing error is caused by inaccuracies and drift of the local timing circuitry with respect to the master clock and can be considerable in power down mode. Herewith, to catch the synchronisation pattern, a width of a correlation window should be considerable too. A broad correlation window, however, can give rise to detection of false synchronisation patterns. Detection of a false synchronisation makes correct synchronisation an even more complex problem.